Scientists studying images of the surface of Mars say they believe they have found NASA's lost Mars Polar Lander spacecraft, which disappeared in 1999 as it attempted to touch down on the planet's surface.

"The loss was a traumatic experience, not only for those of us intimately involved in the mission, but also for the US Mars exploration program," says Professor Michael Malin, of Malin Space Science Systems in San Diego.

Malin is also the principal investigator of the imaging system aboard NASA's orbiting Mars Global Surveyor spacecraft, which may have spotted its fallen cousin.

Mars Polar Lander was to have touched down near the planet's south pole, but the flight team was never able to contact the probe after its scheduled landing time.

Accident investigators later determined the lander's descent engine shut down early, sending the probe crashing to the planet's surface.

Analysing images

Previous attempts to locate the lander's wreckage were not successful.

But recent advances in image analysis techniques created new opportunities for scientists to spot not only the failed lander, but also the Viking probes, which successfully completed NASA's first ground missions on Mars from 1976 to 1980.

Global Surveyor also has returned images of NASA's twin rovers, Spirit and Opportunity, which are in their second years of service on Mars.

It was the images of the rovers that allowed Malin and his colleagues to identify the Polar Lander remains, Malin Space Science Systems says.

"For example, the material from which the Mars Polar Lander and Mars Exploration Rover parachutes are made is similar, and its brightness in [Global Surveyor] images can be calculated, at least in a relative sense, as a function of sun angle," the team writes.

Another similarity is the relative brightness of the ground that was swept by the probes' rocket engines, the scientists say.

Surprisingly, the lander appears to have settled on the planet's surface in one piece.

"We can tell that Mars Polar Lander's descent proceeded more-or-less successfully through parachute jettison and terminal rocket firing," Malin says, adding that judging from the looks of the ground disturbed by the probe's descent engine, the landing appeared normal until the vehicle was close to the ground.

"How close is not known," Malin adds.

"Mars Polar Lander was probably at an altitude of about 40 metres, from which it freely fell."

This is equivalent to a fall on Earth, whose gravity is about three times as much as on Mars, from a height of about 12 metres, Malin says.

"The observation of a single, small 'dot' at the centre of the disturbed location would indicate that the vehicle remained more or less intact after its fall," he adds.

The finding of Mars Polar Lander is more than a footnote to history. The technique used to identify the probe will help scientists using orbiting spacecraft to home in on other small targets of interest.

Malin says follow-up observations, using a higher-resolution observing mode of the Global Surveyor camera, are planned.